1. Field of the Invention
The present invention relates to the transmission, in particular in broadband networks, of data in packets, these packets comprising a header at a first carrier frequency, followed by an information field at a second carrier frequency, the start of transmission of the field being tied to the start of transmission of the header of the corresponding packet.
2. Discussion of the Background
It is known practice to transmit data in packets each comprising a header followed by an information field, the information afforded by the header making it possible, among other things, for the packet to be recognized and taken into account by the parties for whom it is intended.
It is known practice, utilizing a given frequency band for transmitting packets in a network, to define various transmission channels within the band and to allocate them to the transmitters of the network, either in a predetermined manner or on the basis of the transmission requirements; it turns out that such a method does not allow optimal use of the frequency band and is scarcely practical especially when a receiver is liable to receive packets on any of the channels and must therefore monitor the transmission of packets on all the channels.
It is also known practice to assign, within the band of transmission frequencies of a network, a channel to the headers; the receivers need now monitor only this channel and the information provided by the headers makes it possible to receive the fields, the latter being transmitted in the remainder of the useful band on a carrier frequency which may be constant or vary in a predetermined manner, for example in jumps, throughout the duration of the packet.
The invention lies within the realm of the transmission of the field on a carrier frequency which varies. The purpose of the invention is to improve the conditions of transmission, in particular as regards the spreading of the spectrum, the bit rate and the ease of exploitation.
This document deals with symbols. It is recalled that this involves groupings of binary information referred to as bits; these groupings may be expressed under various forms of modulation (amplitude, frequency, phase), each value of the grouping being represented by a state of the constellation in the complex plane. By way of example a grouping of m=3 bits may be expressed through a modulation with N=2m=23=8 phase states, each phase state being situated on the unit circle at multiples of xcfx80/4. Additionally, more particularly, in the case of two-state modulation, the symbol corresponds to one bit.
By using, for the information field, a signal at a carrier frequency which varies according to a linear ramp, this purpose is achieved by modulating the said signal via several subcarriers rather than via one subcarrier, these subcarriers being, themselves, modulated by the data to be transmitted according to a technique of orthogonal frequency division multiplexing, generally referred to as the OFDM technique after its initials. It should be noted that, in what follows, the OFDM technique covers both straightforward OFDM and coded OFDM also referred to as the COFDM technique after its initials; it is recalled in this regard that, considering a binary train, in the OFDM technique the symbols are transmitted in groups of N symbols with N an integer greater than one, respectively on N subcarriers and during a time equal to the time to receive the N symbols. It is recalled that the COFDM technique is merely a variant of the OFDM technique in the sense that, in the COFDM technique, there is moreover associated a coding function which makes it possible to obtain, starting from the N input symbols, N outputs each composed of a weighting of the N input symbols.
According to the invention there is proposed a process for transmitting data in packets, these packets comprising a header at a first carrier frequency, followed by an information field at a second carrier frequency, the start of transmission of the field being tied to the start of transmission of the header of the corresponding packet, characterized in that it consists, for the transmission of the field, in using N, with N an integer greater than 1, distinct and simultaneous subcarriers, in splitting the data to be transmitted in the field into successive groups of N symbols, in assigning the N symbols respectively to the N subcarriers by OFDM multiplexing and in modulating these N subcarriers respectively by these N symbols so as to obtain a modulating signal made of the N subcarriers thus modulated, in generating a signal at the second carrier frequency varying over time according to a linear ramp, in modulating the signal at the second carrier frequency by the modulating signal, and, on reception, in mixing the signal corresponding to the information field with a ramp-like signal similar to the signal at the second carrier frequency so as to obtain a signal corresponding to the modulating signal and to extract therefrom the data of the information field.
According to the invention there is proposed a transmitter for implementing the process, characterized in that it comprises means for computing groups of symbols representative of the data to be transmitted in a field, first means of modulation by the OFDM technique for computing N, with N an integer greater than 1, subcarriers modulated by the groups so as to generate a modulating signal, a ramp generator for generating a carrier signal whose frequency varies over time according to a linear ramp and second means of modulation for receiving the signals generated by the first means of modulation and the ramp generator and for performing a modulation.
According to the invention there is also proposed a receiver for implementing the process, characterized in that it comprises a mixer with a first input for receiving a signal transmitted according to the process, a second input and an output, a ramp generator for delivering, on the second input of the mixer, a signal whose frequency varies linearly over time and, at the output of the mixer, a frequency/time operator followed by a subcarrier demodulation circuit.